Corrosion problem

Commonly used waste heat boilers use flue pipe heat exchange, and the temperature of the lowest wall surface of the metal heating surface is roughly in a multiple relationship with the discharge temperature of the hot fluid. Regarding the smoke tube heat exchanger, if the metal heating surface wall temperature is required to be no less than 150°C, the exhaust gas temperature is usually not less than 300°C, otherwise it will inevitably cause low-temperature condensation and corrosion.

Considering that the equipment is operating at extremely low temperature conditions, the exhaust gas temperature of the waste heat boiler is not less than 450 ℃ based on the safety factor of 1.5 times. At this time, the waste heat boiler can recover about 0.5 tons of heat, and the recovery efficiency is still very low. In addition, the temperature at this time is only a check temperature. When the operating conditions must be adjusted due to operating requirements, there is no way to directly adjust and control the wall temperature.

When the temperature of the metal wall on the heating surface of the waste heat boiler is lower than the condensation point of sulfuric acid vapor, liquid sulfuric acid will be formed on the surface. For a long time, corrosion caused by condensation on the heating surface of the tail of the heat exchange equipment has often occurred. As a result, in the design of the waste heat boiler, the exhaust gas temperature has to be improved or the non-metallic materials with extremely poor heat transfer are used to alleviate the condensation and corrosion, but the problem is still not basically solved. Nevertheless, the waste heat recovery equipment will often show corrosion after one to two years of operation until it is perforated.

The gravity heat pipe waste heat boiler was once implemented, although it can use its isothermal heat transfer characteristics to reduce the exhaust gas temperature to a certain level, but the lowest wall temperature of the heating surface at its tail will still be lower than the acid dew point temperature, which cannot prevent corrosion caused by condensation. In addition, heat pipes generally generate and accumulate non-condensable gas and gradually age, and gravity causes uneven thickness of the heat transfer liquid film to cause unstable heat transfer.

The appearance of the heat transfer technology of the compound phase change toroidal heat pipe has changed this situation. It adopts the principle of heat pipe, puts forward the concept of phase change section, and initiates the new concept of using the wall temperature as the most fundamental design parameter of the heat exchanger. Basically solve the problem of low temperature corrosion. The phase change section handles the problem of low-temperature corrosion, so that the final exhaust gas temperature is infinitely close to the dew point without corrosion, and the purpose of energy saving is achieved. After adjusting the boiling point temperature of the working fluid in the phase change section, the closed-loop control of the lowest wall temperature of the heated surface can be completed, and the effect of constant wall temperature and increase and decrease can be easily completed.

Because the heat pipe is vacuum, the fluid resistance is extremely small, and the distance between the inner and outer layers of the annular heat pipe is only 10 mm, so the heat transfer speed is extremely fast, and a large amount of heat energy is transmitted through its small cross-sectional area at long intervals without any additional power. Due to the common structure of the annular heat pipe, it has been widely used in the fields of thermoelectric industry, chemical and petrochemical industry, power engineering, textile industry, glass industry, and electronic and electrical engineering.

The annular heat pipe is a double-walled dog structure, divided into an inner tube and an outer tube. The annular heat pipe has the following advantages: the heat absorption section is inside the heat release section and can accept higher pressure. The shortest interval between the heat absorption and heat release section, so that the temperature of the medium vapor is reduced to the lowest. Under the same working conditions, the service life is dozens of times that of the ordinary gravity heat pipe. It can be placed at any angle, such as horizontally and diagonally, which brings maximum convenience to product design. The heat absorption and heat release sections are parallel, and the heat transfer speed is several times higher than that of the traditional gravity heat pipe. The starting temperature can be set arbitrarily. When the exhaust gas temperature is lower than the dew point temperature, the system does not absorb heat to prevent the flue gas temperature from falling below the dew point and causing corrosion. Therefore, the final exhaust gas temperature can be infinitely close to the dew point, and the waste heat recovery efficiency is greatly improved.